As a rubber composition for a tire, the technology of compounding silica into a rubber composition is known to obtain both wet performance and low fuel consumption of a pneumatic tire (see, for example, Akimasa Doi, Nippon Gomu Kyokaishi, vol. 71, p. 588 (1998)). As the reason why the silica exhibits such characteristics, it is mentioned that a silica-compounding rubber has a low modulus in the low strain region. However, there are problems that, when silica is compounded, the vulcanization rate becomes slower and the electrical resistance becomes larger, and therefore a large amount of silica cannot be compounded.